1. Field of the Invention
The invention relates to a rack serving device with a lift carriage.
2. Description of the Related Art
A number of different rack serving devices are known, which essentially differ in terms of the layout of the drives and the structural design of the mast, namely depending on whether they are single-mast or twin-mast systems.
Patent specification DE 44 05 952 A1 describes a rack serving device, which is displaced by means of top and bottom drives, the travel of which is controlled in synchronisation. The rack serving device is guided in an aisle and a lift carriage on its mast carries the loads. The rack serving device transmits its own natural weight and the weight of the loads to be carried via a roller with runs along with it in the aisle. In another embodiment of this invention, the mast is supported at the top and bottom region in the direction of the aisle by means of driven gear wheels in stationary gear racks. The synchronised motion is achieved either by electrical synchronisation or by mechanically coupling the two drives. The mast foot and mast head are held vertical one above the other by taking separate measurements of the travel paths and applying a permanent compensation for any deviations by accelerating or decelerating at least one displacement drive. The disadvantage of this rack serving device is the risk of the rack and pinion drive being forced due to misalignment of the mast if the electric synchronisation fails. The uneven moment of inertia of the mast foot and mast head depending on the position of the lift carriage means that greater differences in the braking path between bottom and top traction drives can be expected, and hence misalignment of the mast. In the case of this particular rack serving device, this causes a strong pressure on the sides of the meshing teeth of the rack and pinion drive, which can lead to damage.
Patent specification EP 0 621 231 A1 discloses a rack serving device similar to the one described above, in which the driven gears of the synchronously rotated drives are mounted on universal joints in order to compensate for angular errors and avoid forced pressure.
The same systems with drives acting in a friction lock are also already known from patent specifications CH 518 862 and CH 518 863. The mast is guided on guide rails by means of guide rollers mounted laterally at the mast foot and mast head and is supported in a friction lock in the direction of travel by means of two pinions of a vertical connecting shaft in a respective top and bottom gear rack. The rack serving device transmits the vertical forces into the bottom rail via a flywheel. Since the mast foot and mast head are forcibly guided, there is no need to make allowance for a horizontal gear distance. The disadvantage of this rack serving device is the highly complex mechanical system needed for the connecting shaft and its mounting on the mast, as well as the very high costs incurred by the top and bottom gear rack and their accurate laying in the aisle of the racking system. Furthermore, this rack serving device is susceptible to barely controllable vibrations under dynamic load because the extremely long connecting shaft is very easily twisted and acts like a torsion spring under dynamic influences.
The rack serving devices above known from patent specifications DE 44 05 052 A1, EP 0 621 231 A1, CH 518 862 and CH 518 863 also have problems in terms of noise, lubrication and wear due to the open construction of the teeth and the relatively high cost of the materials, assembly and maintenance. The high build-up of noise also means that the rack serving devices described above can be used on a limited basis only. The various disadvantages outlined above are so serious that these rack serving devices have not been able to gain a foothold on the market to date.
The above-mentioned disadvantages have been overcome by using drives which act on the basis of a friction-locking action. A drive of this type for a rack serving device is known from patent specification DE 44 08 123 A1, which has a drive which can be driven on at least one floor rail and at least one, but as a rule two, load-bearing wheels supported on the floor rail. The load-bearing wheels merely take the load of the entire rack serving device and the driving torque is transmitted to the bogie assembly of the rack serving device by at least one separate friction wheel, which engages on the web of the floor rail. The rails are provided in the form of double-T sections or T-sections, the web of which stands perpendicular to the ground on which the rail is laid and fixed and the load-bearing wheel is supported and rolls on the top and bottom belt of the rails, whilst the friction wheel engages on the web. The friction wheels are forced against the web with a pre-definable contact force. This contact force can be manually set or automatically controlled by means of a runner. The disadvantage of this is that the drive rollers are disposed and roll on oppositely lying guide rails of the web and also serve as lateral guide members for the bogie assembly frame incorporating the drive. If a mast of a rack serving device of this type is fitted with a bogie assembly driven in this manner at its mast foot and mast head and the mast moves out of line from its vertical orientation and/or is subjected to forces acting in the transverse direction to the aisle, the drive rollers will be subjected to increased friction-induced wear, which means that these bogie assemblies can only be used on rack serving devices of a small construction and operated at low displacement speeds. For this reason alone, the range of applications for which rack serving devices fitted with these bogie assemblies can be used is severely limited and they could not be used, or could be used under limited conditions only, for modern order picking systems where travel speeds of up to 7 m/sec and mast heights of up to 20 m as well as low maintenance are required. The deformation occurring caused in the mast during operation alone causes stress on the drive rollers and the increased friction-induced wear mentioned above.
Patent specifications DE 195 34 291 A1 and DE 196 14 660 A1 propose rack serving devices which largely overcome the disadvantages outlined above. These rack serving devices have one mast with a lift carriage which can be displaced on it, which is guided and supported by means of a bogie assembly on the bottom and top rail disposed at the mast foot and mast head, each of which is fitted with a drive. The mast foot and mast head are respectively secured to the bottom and top driven bogie assembly on an articulated bearing, each bogie assembly having two driveable load-bearing wheels acting by means of a friction lock, which sit one behind the other on either side of the articulated bearing so that they can roll on mutually facing top and bottom belts of the bottom and top rails. The load-bearing wheels co-operate with follower rollers which can be applied on opposing sides of the top and bottom belt in order to improve adhesion between the load-bearing wheels and rails. The load-bearing wheels and follower rollers are made from plastic, in particular an elastomer. The disadvantage of rack serving devices of this type is that the driveable wheels are designed to transmit a forward motion to the bogie assembly in the direction of the aisle on the one hand and to take the natural weight of the mast and the load to be carried on the other, which means that high friction forces occur between the load-bearing wheels and the bottom rail, especially when subjected to high loads, and the requisite driving torque of the drive motor has to be significantly increased in equal measure, adding extra costs to the production of the rack serving device and requiring more space in view of the more robust motors needed for the bogie assembly.